It will be noted that an explanation of the two-stage air compressor unit of the prior art type will be made with reference to FIGS. 4a, 4b, and 4c of the drawings. The two-stage compressing air compressor unit of the prior art consists mainly of a motor 4, a two-stage compressor 14 driven by the motor 4, an intercooler 1, and an aftercooler 2. The crankshaft 15 of the two-stage compressor 14 described above is connected to the output shaft 11 of the motor 4 via coupling 16 so that it drives the low pressure cylinder 7 and the high pressure cylinder 8 to compress the air at low and high pressure, respectively. A central protective housing 17 covers the outside of the coupling 16 and shields it from being touched by hand or other objects while it is in rotation. At the same time, the protective covering 17 connects the two-stage compressor 14 to the motor 4. A suction dust filter 18 and a suction muffler 19 are also installed on the respective sides of the central hollow cover 17. The above-mentioned intercooler 1 is between the outlet port 7b of the low pressure cylinder 7 of the two-stage compressor 14 and the inlet port 8a of the high pressure cylinder 8. The air passing through the suction inlet port 7a is compressed at the low pressure cylinder 7 and the higher temperature air is conducted from the inlet port 1a. After some of it is cooled, it leaves via exit port 1b to the suction inlet port 8a of the high pressure cylinder 8. Since the cooling is done by natural heat conduction, the intercooler 1 is located in the good air flow position which is the lower part of or under the two-stage compressor 14.
The compressed air which exits the outlet port 8b of the high pressure cylinder 8 is introduced from the inlet port 2a into the aftercooler 2 which is force air cooled. It is then discharged from the exit port 2b to the brake system of the car which is not shown in FIGS. 4a, 4b, and 4c. Since the cooling is done by the air flow which is put in motion by rotation of the fan 6 which is carried by the other end of shaft 5 of the motor 4, the fan 6 is located on the remote side of the shaft 5 of the motor 4. To install such a two-stage compressing air compressor unit of the prior art underneath the car floor, the suspension pieces 20 are fixed on the upper part of the motor 4 and the two-stage compressor 14. These suspension pieces are bolted to the underside of the car floor with the interposition of a resilient bumper material 21, such as, a vibration-absorbing flexible washer. As shown in FIG. 4c, there are two low pressure cylinders 7 as well as two high pressure cylinders 8. The suctioned air which passes through the suction dust filter 18 and the suction muffler 19 is introduced into each low pressure cylinder 7 from each inlet 7a and is compressed to approximately 2.5 Kg/cm2. Compressed air is conveyed from the outlet ports 7b to the inlet ports 1a of the intercooler 1. From there, it is conducted from each inlet ports 8a and is compressed to approximately 9.0 Kg/cm2. Then, the high pressure compressed air passes through each outlet port 8b and is conveyed to the inlet ports 2a of the aftercooler 2. The aftercooler 2 is force air cooled to not more than 10 degrees higher than the outside air temperature. The compressed air is then supplied to the brake system and other equipment of the railroad car.
Since the two-stage compressed air compressor of the prior art uses natural cooling, the intercooler must be extremely large so that it takes up a great deal of space underneath the car and is expensive to manufacture. Also, the natural cooling intercooler is installed in the vicinity of the two-stage compressor, from which it can easily absorb heat so that the effective cooling capacity of the intercooler decreases which means a loss in energy efficiency. Also, because of its insufficient cooling capacity, the temperature of the compressor tends to rise so that the lubricating oil deteriorates and causes wear of the metal moving parts and an increase in oil consumption results, which makes maintenance and repair operations more difficult.